1. Field of the Invention
The present invention relates to preparation of microparticles containing an active agent. More particularly, the present invention relates to microparticles with a reduced level of residual solvent, and to a method for the preparation of such microparticles.
2. Related Art
Various methods are known by which compounds can be encapsulated in the form of microparticles. It is particularly advantageous to encapsulate a biologically active or pharmaceutically active agent within a biocompatible, biodegradable wall-forming material (e.g., a polymer) to provide sustained or delayed release of drugs or other active agents. In these methods, the material to be encapsulated (drugs or other active agents) is generally dissolved, dispersed, or emulsified in a solvent containing the wall forming material. Solvent is then removed from the microparticles to form the finished microparticle product.
An example of a conventional microencapsulation process is disclosed in U.S. Pat. No. 3,737,337 wherein a solution of a wall or shell forming polymeric material in a solvent is prepared. The solvent is only partially miscible in water. A solid or core material is dissolved or dispersed in the polymer-containing solution and, thereafter, the core-material-polymer-containing solution is dispersed in an aqueous liquid that is immiscible in the organic solvent.
Tice et al. in U.S. Pat. No. 4,389,330 describe the preparation of microparticles containing an active agent by using a two-step solvent removal process. In the Tice et al. process, the active agent and the polymer are dissolved in a solvent. The mixture of ingredients in the solvent is then emulsified in a continuous-phase processing medium that is immiscible with the solvent. A dispersion of microparticles containing the indicated ingredients is formed in the continuous-phase medium by mechanical agitation of the mixed materials. From this dispersion, the organic solvent can be partially removed in the first step of the solvent removal process. After the first stage, the dispersed microparticles are isolated from the continuous-phase processing medium by any convenient means of separation. Following the isolation, the remainder of the solvent in the microparticles is removed by extraction. After the remainder of the solvent has been removed from the microparticles, they are dried by exposure to air or by other conventional drying techniques.
Another conventional method of microencapsulating an agent to form a microencapsulated product is disclosed in U.S. Pat. No. 5,407,609. This method includes: (1) dissolving or otherwise dispersing one or more agents (liquids or solids) in a solvent containing one or more dissolved wall-forming materials or excipients (usually the wall-forming material or excipient is a polymer dissolved in a polymer solvent); (2) dispersing the agent/polymer-solvent mixture (the discontinuous phase) into a processing medium (the continuous phase which is preferably saturated with polymer solvent) to form an emulsion; and (3) transferring all of the emulsion immediately to a large volume of processing medium or other suitable extraction medium, to immediately extract the solvent from the microdroplets in the emulsion to form a microencapsulated product, such as microcapsules or microspheres.
U.S. Pat. No. 5,650,173 discloses a process for preparing biodegradable, biocompatible microparticles comprising a biodegradable, biocompatible polymeric binder and a biologically active agent, wherein a blend of at least two substantially non-toxic solvents, free of halogenated hydrocarbons, are used to dissolve both the agent and the polymer. The solvent blend containing the dissolved agent and polymer is dispersed in an aqueous solution to form droplets. The resulting emulsion is added to an aqueous extraction medium preferably containing at least one of the solvents of the blend, whereby the rate of extraction of each solvent is controlled, whereupon the biodegradable, biocompatible microparticles containing the biologically active agent are formed. Active agents suitable for encapsulation by this process include, but are not limited to, norethindrone, risperidone, and testosterone, and a preferred solvent blend is one comprising benzyl alcohol and ethyl acetate.
U.S. Pat. No. 5,654,008 describes a microencapsulation process that uses a static mixer. A first phase, comprising an active agent and a polymer, and a second phase are pumped through a static mixer into a quench liquid to form microparticles containing the active agent.
U.S. Pat. Nos. 5,792,477 and 5,916,598 (“the Rickey et al. patents”) disclose a process whereby microparticles are contacted with an aqueous washing system to reduce the level of residual organic solvent to less than about 2% by weight of the microparticles. The aqueous washing system is water, or an aqueous solution of water and a solvent for the residual solvent in the microparticles. The aqueous washing system is at a temperature in the range of from about 25° C. to about 40° C. The organic solvent used in such a process is preferably a non-halogenated solvent, and most preferably benzyl alcohol alone or in combination with ethyl acetate.
Because the process disclosed in the Rickey et al. patents uses an aqueous washing system that reduces solvent levels, it suffers from the drawback that it may result in unacceptable depletion of water soluble active agents, such as peptides, from the microparticles.
The documents described above all disclose methods that can be used to prepare microparticles that contain an active agent. None of the documents described above solves the problem of removing residual solvent from microparticles containing a water soluble active agent, particularly when a halogenated solvent is used. None of the documents discussed above discloses a specific method for preparing microparticles that have lower residual solvent levels that is suitable for use with water soluble and non-water soluble active agents, as well as for halogenated solvents. Through the use of the non-aqueous washing system of the present invention, solvent levels can be significantly reduced to acceptable levels, while also maintaining acceptable levels of the active agent.
Thus, there is a need in the art for a method for preparing microparticles having low residual solvent levels for water soluble and non-water soluble active agents. There is a further need in the art for a non-aqueous washing system that can be used to reduce residual solvent levels, for halogenated solvents and non-halogenated solvents. The present invention, the description of which is fully set forth below, solves the need in the art for such methods and system.